A single-handed operation assisting device for a handheld device includes a fixing part for securing a handheld device by clamping, suction, adhesion, or enclosing. In addition, an integrally formed, foldable, or detachable finger straddle part is provided at one side of the fixing part to be straddled between any two fingers, excluding the thumb, of one of the user's hands while the four fingers of the same hand extend downward to support the back of the handheld device so that the handheld device can be held firmly and single-handedly without slipping. As the handheld device is supported at a position adjacent to one of its edges, the user's thumb can reach a farther portion or the lower inner end of the screen with respect to where the handheld device is held, thereby achieving ease of full-screen touch control.